RESEARCH ARTICLE

Spillover of the Atlantic bluefin tuna

offspring from cages in the Adriatic Sea: A

multidisciplinary approach and assessment

Tomislav Dzoić1☯*, Gordana Beg Paklar1☯, Branka Grbec1☯, Stjepan Ivatek-Sahdan2☯,

Barbara Zorica3☯, Tanja Segvić-Bubić4☯, VanjaČikes Keč3☯, Ivana Lepen Pleić4☯,

Ivona Mladineo4☯, Leon Grubisić4☯, Philippe Verley5☯

1 Physical Oceanography Laboratory, Institute of Oceanography and Fisheries, Split, Croatia, 2 Research

and Development Division, Meteorological and Hydrological Service, Zagreb, Croatia, 3 Laboratory of

Fisheries Science and Management of Pelagic and Demersal Resources, Institute of Oceanography and

Fisheries, Split, Croatia, 4 Laboratory for Aquaculture, Institute of Oceanography and Fisheries, Split,

Croatia, 5 Institute de recherche pour le developpement, UMR Botany and Modelling of Plant Architecture

and Vegetation, Montpellier, France

☯ These authors contributed equally to this work.

* dzoic@izor.hr

Abstract

During routine monitoring of commercial purse seine catches in 2011, 87 fingerling speci-

mens of scombrids were collected in the southern Adriatic Sea. Sequencing of the mito-

chondrial DNA control region locus inferred that specimens belonged to the Atlantic bluefin

tuna, Thunnus thynnus (Linnaeus, 1758) (N = 29), bullet tuna, Auxis rochei (Risso, 1810)

(N = 30) and little tunny, Euthynnus alletteratus, Rafinesque, 1810 (N = 28). According to

previously published growth parameters, the age of the collected specimens was estimated

at approximately 30–40 days, suggesting they might have been spawned in the Adriatic

Sea, contrary to the current knowledge. A coupled modelling system with hydrodynamic

(ROMS) and individual based model (IBM—Ichthyop) was set up to determine the location

of the spawning event. Numerical simulations with the IBM model, both backward and for-

ward in time, indicate commercial tuna cages in the middle Adriatic coastal area as possible

spawning location. The two other non-commercial species likely opportunistically use the

positive environmental (abiotic and biotic) conditions to spawn in the same area.

Introduction

Scombridae (mackerels, tunas, and bonitos, [1]) inhabit temperate and tropical seas all around

the world and carry out major feeding and spawning migrations during their life span. Their

worldwide distribution and biological features are well known due to the commercial impor-

tance of these species. Along with mackerels (Scomber colias, S. scombrus) and bonito (Sardasarda), Atlantic bluefin tuna (ABFT), Thunnus thynnus (Linnaeus, 1758), bullet tuna, Auxisrochei (Risso, 1810) and little tunny, Euthynnus alletteratus, Rafinesque, 1810, are widely dis-

tributed in the Mediterranean and Black Seas, as well as in the Adriatic Sea.

PLOS ONE | https://doi.org/10.1371/journal.pone.0188956 November 30, 2017 1 / 20

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OPENACCESS

Citation: Dzoić T, Beg Paklar G, Grbec B, Ivatek-

Sahdan S, Zorica B, Segvić-Bubić T, et al. (2017)

Spillover of the Atlantic bluefin tuna offspring from

cages in the Adriatic Sea: A multidisciplinary

approach and assessment. PLoS ONE 12(11):

e0188956. https://doi.org/10.1371/journal.

pone.0188956

Editor: Aldo Corriero, Universita degli Studi di Bari

Aldo Moro, ITALY

Received: September 15, 2016

Accepted: November 16, 2017

Published: November 30, 2017

Copyright:© 2017 Dzoić et al. This is an open

access article distributed under the terms of the

Creative Commons Attribution License, which

permits unrestricted use, distribution, and

reproduction in any medium, provided the original

author and source are credited.

Data Availability Statement: All relevant data are

within the paper and its Supporting Information

files.

Funding: This work was supported by the Croatian

Science Foundation: IP-2014-09-9050 (www.izor.

hr/aquapop). The authors who received the funding

– TSB, LG. It was also supported by the Croatian

Science Foundation: IP-2014-09-3606 (http://

jadran.izor.hr/~kovac/mariplan/). The authors who

received the funding – GBP, TDZ, BG. Croatian

The Atlantic bluefin tuna (Thunnus thynnus thynnus, Linnaeus, 1758) is a large, highly

migratory, pelagic predator displaying seasonal migrations and spawning site fidelity, in both

the Mediterranean Sea and the Gulf of Mexico, which constitute its two main spawning areas

[2]. Global decline of wild bluefin tuna populations as a result of heavy fishing pressure [3, 4]

makes farming an attractive alternative to fisheries, although capture-based farming, a widely

adopted type of bluefin production, requires a comprehensive approach to ensure sustainabil-

ity. Namely, as a top predator, tuna requires large amounts of highly caloric food (herrings,

sardine), which increases operational costs and space requirements, while tuna breeding in

captivity, although feasible, is still hampered by various zootechnical bottlenecks, impeding its

employment on a large scale [5].

In Croatia, tuna farming is based on the capture of smaller wild tunas (8–10 kg) and their

subsequent farming for up to 36 months to market size (�30 kg). Farming is performed in

floating cages at sea with an annual production of up to 4,000 tonnes, exported almost entirely

to the Japanese market [6]. Five semi-off shore farming locations run by four different compa-

nies are located in the central eastern Adriatic.

Considering the need for consistent tuna supply and a reduction of impacts on wild-

caught juveniles, strong scientific collaboration was established in the early 2000s aimed at

closing the life cycle of ABFT in Europe. Broodstock domestification and hormonal spawn-

ing induction protocols were successfully carried out, leading to the development of larval

rearing technologies and formulated diets [7]. Furthermore, spontaneous spawning events

of ABFT in captivity have been observed throughout the Mediterranean [8–10], including

at Croatian tuna farms in the middle Adriatic where spawning behaviour was observed in

the summer months [11].

Spawning of natural stocks of Atlantic bluefin tuna, bullet tuna and little tunny in Mediter-

ranean Sea (Balearic Islands, coasts off Sicily, Libya and Cyprus) [12] occurs during the

warmer part of the year; bluefin tuna from May to July [8, 13, 14], bullet tuna from June to Sep-

tember [14–16] and little tunny from May to September [17]. In addition to the observed hom-

ing behaviour of these spawning grounds, there is conflicting data published in non peer-

review papers concerning the Adriatic Sea as a potential bluefin tuna spawning ground. New

ecological niche modelling approach [13] has recognized only the central Ionian as a second-

ary potential spawning ground, where recent electronic tagging experiment [18] indicated this

possibility. Within the same paper the Adriatic Sea was mentioned as feeding ground due to

results of tagging.

Furthermore, incidental catches of tuna-like juvenile specimens, whose stocks are under

strict regulation and whose spawning in the Adriatic Sea south of the island Mljet has not been

previously observed, could greatly contradict our perceived knowledge on the species ecology,

and may affect future decision making in stock management. Namely, 87 fingerling specimens

of scombrids were collected during routine monitoring of commercial purse seine catches in

September 2011 in the southern Adriatic. Morphological and molecular species identification

implied the feasibility of tuna spawning in the Adriatic Sea for the first time. Upon identifica-

tion of tuna and tuna-like species, the aim of this study was to determine potential spawning

grounds of bluefin tuna through application of the coupled modelling system. This multidisci-

plinary study included a description of collected data, and results of genetic and phenotypic

analyses, supported by a numerical framework of Regional Ocean Modeling System (ROMS)

[19, 20] and Individual Based Model (IBM) Ichthyop [21]. Additionally, prevailing meteoro-

logical conditions and output of both models, along the comparison to available measure-

ments, were described in details.

Spillover of the Atlantic bluefin tuna from cages in the Adriatic Sea

PLOS ONE | https://doi.org/10.1371/journal.pone.0188956 November 30, 2017 2 / 20

Science Foundation: 001-0013077-0532 (http://

www.izor.hr) also supported this work. The

authors who received the funding - BZ, VČK. Also

project PERIMON is added to the list of projects

that financially supported our study. The author

who received the funding – VČK. The funders had

no role in study design, data collection and

analysis, decision to publish, or preparation of the

manuscript.

Competing interests: The authors have declared

that no competing interests exist.

Materials and methods

Morphological and molecular identification

Unidentified scombrid specimens (N = 87) were collected on 1 September 2011 by authors of

this article during routine monitoring of commercial purse seine catches along the eastern

Adriatic Sea, at the Croatian fishing ground in the open sea area off coast of the island Mljet

(42˚38’55.32”N, 17˚44’31.00”E) (Fig 1). This monitoring was conducted as part of the PERI-

MON project, financed by the Croatian Ministry of Agriculture, Department for Fisheries.

Sampling was conducted on commercial species, which are not endangered or protected, with

common fishing gear and grounds, and therefore no specific permits were required. Further-

more, animals collected from commercial fisheries with methodology normally used in fishery

practices were not exposed to any unnecessary pain, injuries or suffering. Collected specimens

came from the same haul and were immediately frozen, fin clips stored in absolute alcohol for

molecular identification and transported to the laboratory for subsequent analysis. In the labo-

ratory total length (TL), fork length (FL) (±0.1 mm) and total body weight (W) (±0.01 g) of

each fish were measured (S1 Table).

Fig 1. Adriatic Sea bathymetry with locations of the ABFT catch (island of Mljet) and fish cages (near islands of Ugljan (44˚2’27.00’’N, 15˚

10’48.45’’E), Fulija (44˚1’23.00”N, 15˚6’31.55’’E), Gira (43˚50’11.37’’N, 15˚29’34.54’’E) and Brač (43˚17’19.93’’N, 16˚28’53.23’’E)). Contours are

drawn for 250, 500, 1000 and 1200 m depths. Blue squares are depicted around bluefin farming cages, representing the area of success for particles

in the backward experiment. The black square is depicted around the natural environment of catch location, representing the area of success for

particles in the forward experiment. Green dots represent coastal sea surface temperature stations (Split, Hvar, Komiza). Four black dots at the map

of the entire Adriatic (upper right corner) represent points of release in the Strait of Otranto (40˚11’46.32’’N, 18˚35’7.8’’E; 40˚41’20.76’’N, 19˚

13’58.44’’E; 40˚31’48.36’’N, 19˚1’24.96’ E and 40˚21’18.72’’N, 18˚47’37.68’’E) (Experiment 10, Table 1).

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Molecular identification. Genomic DNA was extracted from 10–50 mg of tissue using

the DNeasy Tissue Kit (Qiagen Inc.) following the manufacturer’s protocol. A partial fragment

of the mitochondrial DNA control region was amplified following Grubisić et al. 2013 [11].

Products sequencing were performed by Macrogen Inc. (Seul, Korea) on an ABI 3730 auto-

matic sequencer. The obtained sequences (~800 bp) were compiled along sequences of other

scombrids available in GenBank (T. thynnus AB106300.1, A. rochei AB107063.1, E. alletteratusAB099716.1) in MEGA 6 and aligned incorporated by Clustal X 1.83 [B] using default parame-

ters. DnaSP 5.19 software [22] was applied to calculate haplotype diversity. Phylogenetic rela-

tionships among mtDNA haplotypes of scombrids were reconstructed using Neighbour-

Joining analyses and p-distance calculation in MEGA 6. A bootstrap test with 1000 iteration

values was performed on the unrooted NJ tree.

Morphometric traits. For each species, conversions between length measures were

accomplished with linear regression models using the least squares method, while the length-

weight relationships were calculated by applying standard exponential regression equation. In

order to obtain the age of the collected specimens previously published growth parameters

were used as follows:

• Atlantic bluefin tuna, Thunnus thynnus (Linnaeus, 1758): FL = 41.20+2.37t (FL:fork length

in mm, t:age in days; [23]),

• bullet tuna, Auxis rochei (Risso, 1810): FL = 29.74(1-e(-10)(t-0.018)) (FL: fork length in mm, t:age in years; [24]),

• little tunny, Euthynnus alletteratus, Rafinesque, 1810: FL = 2.036+0.396t (FL: fork length

in mm, t: age in days; [25]).

Description of the modelling system

In order to determine the possible location of the spawning event, a coupled modelling system

ROMS—Ichthyop was set up and run. Hourly averaged current, temperature and salinity fields

were calculated using the Regional Ocean Modeling System (ROMS) [19, 20] for the period

from 17 July to 2 September 2011. Detailed description of the ROMS model, its set up and

evaluation of the results are given in S1 Appendix. Surface circulation of the Ionian Sea, used

in the discussion, was assessed from the physical reanalysis component of the Mediterranean

Forecasting System which was obtained from the Copernicus Marine Environment Monitor-

ing Service database.

Ichthyop v.3.3.a (http://www.ichthyop.org/downloads; [21]) is a tool that uses physical

(temperature, salinity, currents) and biological (egg buoyancy, diel vertical migration, swim-

ming, larvae growth, mortality related to temperature, etc.) parameters to give an estimate on

icthyoplankton dynamics. The time step in the IBM simulations was 600 seconds, and bounc-

ing coastline behaviour was applied. The numerical Runge-Kutta 4th order scheme was used

for advection. Both forward and backward simulations were performed to gain more informa-

tion regarding particle dispersion and to determine possible spawning location. Decision to

use 20000 particles as an optimal particle number in experiments was based on the statistic sta-

bility criterion [26]. Passive particle displacement determined by modelled currents and vari-

able diffusivity fields was used in backward simulations with no additional options.

The starting point for simulations backwards in time was near Mljet, where particles were

released from the stain with a radius of 500 m and thickness of 15 m in depth (Experiment 1, see

Table 1). Simulations lasted 46 days back in time after particles were released on 1 September.

Additional depth ranges based on diel larvae distribution studied near the Balearic Islands [14]

and thermocline level (S1 Appendix) were tested too (Experiments 2, 3 and 4). In Experiment 4

Spillover of the Atlantic bluefin tuna from cages in the Adriatic Sea

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thermocline level is used as a minimum expected depth because at age of collected specimens,

ABFT is an active predator and prey could be below the thermocline. As it was assumed that col-

lected small tuna spawned in the fish farm, sources of particles for forward simulations were the

fish farms near Ugljan (this also represented the farm at Fulija due to their proximity which

placed them both within the same numerical cell), Gira and Brač (Fig 1). The stain radius was

250 m to encompass the area similar to cages, while its vertical extension was within the first 15

m where most tuna are located during spawning cycles, although the cages protrude to a depth

of 30 m. Each day between 17 July and 5 August at 0300 UTC 1000 particles were released from

each cage location. Sensitivity experiments indicate that the use of any other release time during

tuna spawning hours between 0000 and 0300 UTC [9], apart from the selected ending time, has

no significant impact on dispersion results. Forward experiments were performed with passive

particle displacement only (Experiment 5), with passive displacement combined with active

swimming (Experiments 6 and 7) and with active swimming only (Experiments 8 and 9). Parti-

cle swimming velocity was one, two or four body lengths per second [27, 28], where body length

was a function of age based on the previously stated published growth parameters. Swimming

direction was randomly chosen in each time step. Two possibilities regarding swimming were

used: constant speed implies that particles swim at the velocity defined as a function of age, while

randomly chosen speed means that particles swim at a random speed ranging from zero to the

velocity defined as a function of age. As the incubation period for eggs in captivity is 30 hours

[11], this phase was neglected in the 48-day numerical experiments, and the assumption that par-

ticles had the initial length of newly hatched larvae and possibility to swim was used. An addi-

tional forward experiment (Experiment 10) was performed to test the pathways of particles

originating from four points at the Strait of Otranto (Fig 1).

Results

Molecular identification and growth performance

Sequencing of the mitochondrial control region locus inferred that specimens belonged to the

Atlantic bluefin tuna, Thunnus thynnus (Linnaeus, 1758) (N = 29), bullet tuna, Auxis rochei(Risso, 1810) (N = 30) and little tunny, Euthynnus alletteratus, Rafinesque, 1810 (N = 28).

Namely, phylogenetic reconstruction showed the presence of three distinct clades (Fig 2) cor-

responding to the presence of these three scombrid species. Thunnus thynnus was presented

with 20 haplotypes (GenBank acc. no. KY491009 –KY491028), Auxis rochei with 23 haplotypes

Table 1. List of numerical experiments.

Experiment

number

Orientation in

time

Depth of

release (m)

Point of

release

Passive particle movements

(advection plus diffusion)

Swimming (none, constant speed,

randomly chosen speed)

Experiment 1 Backward 0–15 Mljet Yes None

Experiment 2 Backward 15–30 Mljet Yes None

Experiment 3 Backward 30–50 Mljet Yes None

Experiment 4 Backward 20–50 Mljet Yes None

Experiment 5 Forward 0–15 Cages Yes None

Experiment 6 Forward 0–15 Cages Yes Constant speed

Experiment 7 Forward 0–15 Cages Yes Randomly chosen speed

Experiment 8 Forward 0–15 Cages No Constant speed

Experiment 9 Forward 0–15 Cages No Randomly chosen speed

Experiment 10 Forward 0–50 Strait of

Otranto

Yes None

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(KY446395—KY446417) and Euthynnus alletteratus with 14 haplotypes (KY446418—

KY446431).

Length frequency distribution of each analysed scombrid species is given in Fig 3. Fork

length range of Atlantic bluefin tuna specimens was somewhat narrower (10.8<FL<15.0 cm;

mean±SD: 12.7±1.09 cm) than for bullet (13.2<FL<21.8 cm; mean±SD: 16.1±2.27 cm) and lit-

tle tuna (11.6<FL<18.4 cm; mean±SD: 13.6±1.73 cm). Accordingly, the measurable range and

Fig 2. Unrooted NJ tree based on the p-distance of the mtDNA control region sequences used. Bootstrap values are shown along the

branch for each clade. Haplotypes (H) of Thunnus thynnus (Tt), Auxis rochei (Ar), Euthynnus alletteratus (Ea) are presented.

https://doi.org/10.1371/journal.pone.0188956.g002

Spillover of the Atlantic bluefin tuna from cages in the Adriatic Sea

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mean total body weight of Atlantic bluefin (16.97<W<53.45 g; mean±SD: 30.74±9.356 g) was

lower than for bullet (23.83<W<148.88 g; mean±SD:55.74±31.837 g) and little tuna

(12.94<W<93.21 g; mean±SD:35.45±18.465 g).

Conversions between total (TL) and fork (FL) length and the length-weight relationship for

each studied species are given in Table 2. All obtained length–length regressions and length-

weight relationships were statistically significant (P< 0.001) with high values of the correlation

coefficient (0.967� r� 0.999). Statistically significant positive allometric growth was estab-

lished for all three species (Atlantic bluefin tuna: t-test = 241.6, df = 28, P<0.0001; bullet tuna:

t-test = 82.7, df = 27, P<0.0001; little tunny: t-test = 88.6, df = 29, P<0.0001).

According to previously published growth parameters, the age of each scombrid species

was estimated (S1 Fig). In general, the overall scombrid age range was between 14.8 (bullet

tuna FL = 13.2 cm) and 45.9 (Atlantic bluefin tuna FL = 15 cm) days, while the average age of

all studied specimens was 29.5±7.02 days.

Meteorological and oceanographic conditions

Detailed analysis of prevailing meteorological and oceanographic conditions was performed

using all available data. During the study period weather was under the well-developed Azores

high and Karachi depression [29], which over the Adriatic produce warm and almost dry

weather with characteristic a well-developed sea-land breeze system. Daily air temperature

means at 2 m height from the ALADIN model never dropped below 24˚C on the open sea dur-

ing the entire study period (Fig 4). Daily wind vectors at 10 m height blew from varying direc-

tions, though the most frequent were the northwestern and western winds (Fig 4). SST at Split

Fig 3. Length frequency distribution of Atlantic bluefin tuna, Thunnus thynnus (Linnaeus, 1758), bullet tuna, Auxis

rochei (Risso, 1810) and little tunny, Euthynnus alletteratus, Rafinesque, 1810 collected with commercial purse

seiners operating in the southeastern Adriatic, September 2011.

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Table 2. The length–length (between total (TL) and fork (FL) length) and length-weight (fork length (FL)–total body weight (W)) relationships sepa-

rately given for each scombrid species, Adriatic Sea, September 2011.

Species TL-FL FL-W

Atlantic bluefin tuna FL = 0.9424TL-0.456 W = 0.0039FL3.5152

Bullet tuna FL = 0.9517TL-0.016 W = 0.0035FL3.4435

Little tuna FL = 0.9283TL+0.164 W = 0.0019FL3.7387

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and Hvar coastal stations in July and August 2011 was over one standard deviation above nor-

mal and at Komiza SST was two standard deviations above normal (Fig 5, available at http://

www.izor.hr/web/guest/virtual-laboratory; [30]). The most prominent difference between

ROMS and the satellite SST was seen in the open parts of the middle and southern Adriatic,

though discrepancies were within values that could not impact the lower threshold of spawn-

ing temperature (Fig 6). Negative model discrepancies in front of the Adriatic coast indicate

that we need to improve river implementations in the model. River discharges are determined

from scaled climatological values and their temperature effect is neglected at the moment.

More realistic river implementation is expected to improve modelled temperature fields.

Strong temporal changes of surface temperature discrepancies in Fig 6C indicate possible

effect of synoptic conditions [29], during which impact of different processes dominates. This

issue needs further analysis which is beyond the scope of our paper. Anyway, we believe that

Fig 4. Daily mean of air temperature at 2 m and wind field at 10 m height obtained from ALADIN on 22 July (a), 8 August (b) and 25 August (c).

Wind vectors are plotted at every ninth grid point.

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Fig 5. Time series of the available SST values averaged for July and August in the period between

1950 and 2015 at the coastal sea surface temperature stations Split, Hvar and Komiza.

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for simulations we performed with Ichthyop model, mostly with passive particles, obtained

discrepancy maxima of 2˚C has no significant impact for the final results. Particularly because

areas with maximum discrepancy are out of the assumed tuna paths. Apart from satellite SST,

additional evidence of favourable spawning temperature conditions was the directly measured

seawater temperature at 1 m depth inside the cage near the island of Ugljan (Fig 7) (S2 Table).

Differences between measured and modelled subsurface temperatures show two patterns:

before 29 July with similar values and after 29 July with similar trends but with higher discrep-

ancies in the values. Obtained behaviour can be related to different synoptic conditions before

and after 29 July. From 14 to 18 July and from 30 July to 15 August the weather was stable,

warm and dry, while from 19 to 29 July atmospheric instabilities occasionally passed over the

Adriatic [29]. Obtained modelled values were equal or over the measured values and during

stable conditions model obviously heats too much. Although maximum discrepancies of 1.5˚C

are satisfactory for the simulations with passive particles, this issue requires further analysis.

Moreover, we expect that higher horizontal resolution in the ROMS model simulations would

increase model accuracy. Measured point is very close to the coast and it is hard to expect that

Fig 6. Daily mean SST obtained from reprocessed AVHRR satellite data (column a), daily mean SST obtained from ROMS

(column b) and difference between ROMS and reprocessed AVHRR SST (column c). The first row shows data for 23 July, second row

for 14 August and third row for 1 September 2011.

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2 km resolution model is capable to reproduce precisely temperature close to such irregular

coastline with many island. Higher horizontal resolution will be used in the future simulations

in which we are planning to include growth and mortality temperature dependences. Anyway,

with noted correlation between the model and measurements for cage location near the island

of Ugljan and from simulated temperatures near the islands of Brač and Gira, it could also be

assumed that spawning may have also occurred at these islands. A seawater temperature of

19.5˚C is the lower threshold for wild tuna spawning in open seas of the West Mediterranean

[9]. A sharp and sudden rise in temperature to 25˚C recorded during spawning in captivity

could be an additional favourable condition [31]. Similar behaviour was also observed in the

present study (Fig 7) [11]. Modelled surface currents and currents at 10 and 30 m were in the

opposite direction to the northwesterly East Adriatic Coastal Current, which is a part of the

Adriatic basin-wide cyclonic circulation [32]. A similar change in summer flow direction

along the eastern Adriatic coast was previously reported from direct current measurements

[33] and numerical model results [34]. Modelled temperature remained above 19.5˚C in the

top 10 m along the presumed pathway of small tunas on the open-sea side of the Croatian

islands.

Fig 7. Modelled and measured time series of seawater temperature at 1 m depth within the fish farm cage near the island of Ugljan (a),

and modelled time series of seawater temperature at 1 m depth within the fish farm cages near the islands Gira and Brač (b).

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Distributions of particles in backward and forward IBM simulations

In the backward simulations only passive “Lagrangian” experiments were performed in order

to assess whether particles could successfully reach the areas around the three studied fish

farms. Spatial distributions for Experiment 1 on 5 August (Fig 8A) and 19 July (Fig 8B) indi-

cated that particles after 28 and 46 days arrived near the location of the fish cages off the island

of Brač only. Spatial distributions are shown for 0300 UTC to match the daily end of the tuna

spawning period and were calculated by counting the number of particles within the 2.5x2.5

km ROMS grid cell. The area of success for backward simulations was defined as a square of

225 km2 area where the fish cage was centrally situated. Due to the uneven distribution of land

and sea at these locations, there was a need to estimate a weighting factor to obtain more reli-

able results for success at all three locations. Therefore, the percentage of sea points within the

square with the tuna farm was multiplied by the number of particles within the square. The

time series for Experiment 1 of particle arrivals near Brač (Fig 9) indicated that the number of

particles increases by moving back in time with the first date of spawning. Experiments in

which particles were released deeper (Experiments 2, 3 and 4) yielded similar results as Experi-

ment 1. Reason for this is prevailing southeasterly current in the first 50 m of depth resulting

in northwestward transport in the backward simulations (S1 Appendix).

In the forward simulations, the area of success was defined as a square of 625 km2 area

where the sampling site was located centrally. The main reason for the larger area of success in

the forward simulations than in backward simulations was the intention to catch more parti-

cles, as the estimated age of captured tuna suggested that they developed the ability to swim.

Particle distribution on 1 September (Fig 10) clearly showed that some of particles arrived in

the square near the island of Mljet. The spatial distribution on 1 September was quantified by

calculating the number of particles in the area of success in respect of the point of origin and

the result (Fig 11A) was the same as in the backward experiment, confirming that they origi-

nated only from Brač. Dates of their release from cages encompassed the entire assumed

spawning period (Fig 11B).

To assess the effect of active swimming four additional forward, experiments were per-

formed (Table 1; Experiments 6, 7, 8 and 9). Particle distribution on the 20th day after release

Fig 8. Spatial distributions of particles obtained in the backward Ichthyop experiment with 20000 passive tracers on 5 August (a) and 19

July (b). Origin of particles is represented by a black dot. The area of success is shown by black rectangles near the three fish farm cages.

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(Fig 12) showed that particles from Brač could be transported near the catch location even as

the passive tracers (Table 1, Experiment 5). Active swimming was first tested as the only

means of moving particles and the result showed that the distribution of particles encompassed

a radius of 50 km for swimming speed of one body length/second (Fig 13A) and radius of 70

km for swimming speed of four body length/second (Fig 13C) Combination of both swim-

ming options and passive movements (Experiments 6 and 7) yielded that particle distributions

for both used swimming speeds (Fig 13B and 13D) were similar to particle distributions with

passive displacement only (Experiment 5). The reason for such small deviation from the distri-

bution based on passive movement is mainly the fact that the direction in which particles are

swimming is randomly chosen in each time step.

To test the possibility of particles arriving from the Ionian Sea at the sampling site, in

Experiment 10 particles were released from the southern edge of the domain (Fig 1). At the

end of simulation, all particles were transported outside the model domain towards the Ionian

Sea due to the prevailing southeasterly surface flow.

Discussion

In a fluctuating ecosystem, such as the sea, sustainability of fish population is strongly deter-

mined by successful reproduction. Hence, knowledge on fish reproduction and its ecologically

important habitats, such as spawning and nursery grounds, is required to allow scientific advi-

sors and regulators to better manage their stocks.

An incidental finding of scombrids in September 2011 in the southern Adriatic raises the

issue of the possible tuna spawning in the Adriatic Sea. In addition to the scientific signifi-

cance, strict legislative regulation of tuna stock concerning farming and fisheries, is additional

motivation to resolve this dilemma.

The main goal of this study was to discover the origin of small tuna fish caught in the south-

ern Adriatic. Therefore, a multidisciplinary investigation was undertaken encompassing purse

seine monitoring data, their genetic and phenotypic analysis, analysis of the prevailing meteo-

rological and oceanographic conditions and finally the numerical study of the early tuna stage

dynamics using coupled modelling system ROMS—Ichthyop. Similar approach with genetic

analysis and circulation modelling used to investigate pan-Atlantic connectivity of endangered

green turtles [35] is another example of multidisciplinary investigation widely used nowadays

Fig 9. Time series of the particle arrivals within the rectangle of success around the Ugljan (red line), Gira (black) and Brač (blue) tuna

farms. Particle numbers are weighted by the percentage of wet points inside the rectangles of success for the backward experiment with 20000

passive tracers.

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in revealing population structures and their spatial distributions. Although genetic and pheno-

typic analyses were performed for all collected specimens, further numerical experiments and

discussions were focused on bluefin tuna.

Phenotypic analyses (length-length relationship and positive allometric growth) were in

line with previously published data for each analysed scombrid species [23] and the obtained

age for all collected specimens (between 30–40 days) suggested their possible spawning in the

Adriatic. There are several possibilities for the spawning location of the collected small tunas.

The closest known area of natural tuna spawning to the sampling site is the eastern Sicilian

coast [12, 13]. An average surface current of 20 cm/s obtained during August 2011 by the Med-

iterranean model (Fig 14) along the possible tuna pathway and ability of tuna to swim 20 days

after spawning with speeds ranging from 8 cm/s on 20th to 15 cm/s on 48th day would allow

for crossing about 700 km within 35 days from the eastern Sicilian coast to the sampling site.

Fig 10. Spatial particle distribution obtained in the forward Ichthyop experiment with 20000 passive tracers from each source located

near fish farms (black dots) on 1 September. The black rectangle represents the area of success around the location of the ABFT catch (red

dot).

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Multiplying swimming speeds with factors ranging from 2 to 4 would allow further easier and

faster arrival from the eastern Sicilian coast to the island Mljet. However, the prevailing south-

erly direction of the surface flow and its complex structure most likely create an unsurpassable

obstacle for small tunas to reach the southern coast of the island Mljet. An additional argument

to assess the eastern Sicilian coast as an unlike spawning location is the agreement of the south-

erly surface flow from the Mediterranean model with the ROMS model results used by

Ichthyop. Moreover, prevailing southerly surface flow transported all particles in Ichthyop

simulation originating from the Strait of Otranto towards the Ionian Sea instead towards sam-

pling site. Since natural tuna spawning in the Adriatic is questionable, the working hypothesis

assumed that the discovered small tunas spawned in one of the farms positioned along the

eastern Adriatic coast north of the sampling site. Upon spawning, they were carried south-

wards by the prevailing SE current for about 20 days. After approximately 20 days, the tuna

could actively swim independent of the hydrodynamic environment [36], allowing them to

arrive at the sampling place within their estimated age.

An important part of the study was the numerical modelling system, which enabled the test-

ing of the hypothesis on the spawning location. A similar modelling setup with a hydrody-

namic and individual based model (IBM) has been extensively used for the early stages of

Fig 11. Total particle numbers within the rectangle of success obtained in the forward Ichthyop experiment with 20000 passive tracers from

each source located near the fish farm on 1 September depending on the point of origin (a). Distribution regarding the age of particles within the

rectangle of success obtained in the same experiment for the point of origin Brač (b).

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Fig 12. Spatial particle distribution on 20th day after release obtained in the same experiment as in Fig 10 but only for the source located

near the fish farm Brač (black dot). Day of snapshot: 8 August (a), 14 August (b) and 16 August (c). Black rectangle represents the area of success

around the location of the ABFT catch (red dot).

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many species (sardines, anchovy), though published data and modelling of early stage dynam-

ics of tuna are scarce. According to our knowledge, this study is the first attempt to numeri-

cally model early stage dynamics of tuna in the Adriatic Sea. Using hourly ROMS output and

horizontal resolution of 2.5 km we preserved all important physical processes which occur in

the Adriatic [37]. Anyway, taking into account complex topography further improvements

could be made for the simulations along the eastern Adriatic coast by applying finer spatial res-

olution and nesting procedures [34]. Due to the lack of information and knowledge of the pro-

cesses during the early life of tuna, numerical simulations were conducted with the

assumption that particles in the Ichthyop model were passive. Introduction of additional pro-

cesses and dependences of the tuna eggs and larvae in the simulations, such as buoyancy, diel

vertical migrations, temperature dependences, would likely increase the model accuracy and

reliability of the results. Some processes are already included in the Ichthyop simulations,

Fig 13. Spatial particle distribution obtained in the forward Ichthyop experiment with 20000 particles from the source near the island of

Brač on 1 September resulted only from swimming ability of particles with swimming speed of one body length/second (a) and

swimming speed of four body length/second (c). The difference on 1 September between distributions obtained in the forward Ichthyop

experiment with 20000 passive tracers from the source located near the fish farm Brač (black dots) with advection and diffusion only, and

distribution with the same point of origin and same number of particles with advection, diffusion and swimming speed of one body length/second (b)

and of four body length/second (d).

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Spillover of the Atlantic bluefin tuna from cages in the Adriatic Sea

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though most of the parameterizations required have been obtained for areas outside the Adri-

atic. The presently introduction of the Ichthyop default diel vertical movement [21] resulted in

a pile up along the western Adriatic coast during the forward experiment. Therefore, in order

to proceed with similar numerical investigations, better parameterizations from the laboratory

and field experiments in the Adriatic are needed. ARGO drifter data (S1 Appendix) were used

in our study for verification of the ROMS reproduced circulation. Unfortunately it was not

possible to use the drifter data for assessing the connectivity between supposed spawning loca-

tion and sampling site and comparison between passive and active movements since ARGO

drifter was moving in the Jabuka Pit far away from the presumed tuna pathway [38].

Furthermore, the present study clearly showed the importance of accurately simulated

physical fields, particularly the current field responsible for the early tuna stage transport. Real-

istic numerical simulations revealed a current reversal along the eastern Adriatic coast, from a

NW to SE flow. Occurrence of the current reversal has previously been observed [33] and

modelled [34] in this area, particularly during summer. The impact of reversal has never been

studied with dispersion simulations to reveal the area connectivity and its impact on marine

ecology. Connectivity studies in the Adriatic Sea were more focused on decadal time scales

and basin as a whole [39, 40]. Without the reversed SE flow, tuna eggs and larvae would not be

able to travel south to the sampling site. With a NW flow, small tuna would move northwards

of the fish farms in the middle Adriatic and it is questionable whether they would survive in

the northern coastal area of the Adriatic. Reproduction of the temperature field is also very

important as the actual onset of spawning behaviour is temperature related [9].

Results of the numerical simulations with the Ichthyop model, both backward and forward,

indicate that the commercial tuna cages in the middle Adriatic coastal area are a possible

spawning location, while the tuna farm off the coast of the island of Brač was the most probable

origin of the collected specimens. Due to the presence of the middle Adriatic gyre, passive par-

ticles in the backward experiments could not reach the other tuna farms at Gira and Ugljan,

thus eliminating them as a possible origin, despite records of natural spawning in these cages

[11]. The forward experiment indicated that the particles from the Brač tuna farm could arrive

20 to 50 km away from the sampling site within 20 days even as passive tracers no matter of

releasing date. This result indicates that prevailing current direction is the most important fac-

tor for arrival of small tunas at the sampling site and sensitivity on time and date of release is

not precedent [41]. We are aware that passive displacement by currents only and with addition

of swimming with randomly chosen direction was not the best choice, but in the lack of better

model swimming parameterizations and knowledge of tuna swimming, it represented a usable

Fig 14. Daily mean sea currents calculated by the Mediterranean model at 1m depth on 23 July (a) and

19 August (b). Vectors are plotted at every fourth grid point. A white rectangle denotes the area with

maximum speed during the selected date.

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Spillover of the Atlantic bluefin tuna from cages in the Adriatic Sea

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tool to study small tuna displacement. Even if this displacement was the slowest one and in

most cases excluded maximal displacement possible that could arrive from matching the direc-

tions of swimming and prevailing currents. By adopting the hypothesis that the Brač tuna farm

is the spawning location, we have valuable material for future testing of numerical model

improvements. It is very rare to have exact location of spawning. Typically, this is a larger area

with a high possibility of spawning, whereas in this case, we have virtually a point source in

which spawning occurred (fish farm cage). A well-defined spawning location gives high confi-

dence in assessing both model results, hydrodynamic ROMS and IBM Ichthyop.

Conclusions and future work

Finally, it can be concluded that based on the performed simulations, the most probable

spawning location for the Atlantic bluefin tuna found in the Adriatic was the commercial tuna

cages off the southern coast of Brač Island. This is possible, as natural spawning has previously

been reported within commercial tuna farming facilities in the Adriatic [11]. However, the fact

that the two other species most likely opportunistically use the positive environmental condi-

tions and spawn in the same area could led us to further investigate the possibility that bluefin

tuna could do the same.

This multidisciplinary study, supported by genetic and phenotypic analysis, is the first

attempt to numerically model early tuna stage dynamics in the Adriatic Sea. There are few sim-

ilar numerical investigations for other areas, and therefore it was impossible to include more

biology of the tuna early stage without increasing the uncertainty in the model solutions.

Numerical experiments were performed with passive particles, although modelled specimens

do not fall entirely into passive swimmers. The effects of active swimming were tested in the

forward experiments. Active swimming implemented with passive movements, with the

assumption of randomly chosen swimming direction in each time step, had no significant

influence on the IBM results. Additional testing should be made for swimming patterns paral-

lel or perpendicular to the prevailing current orientation [42].

Many improvements are possible for the hydrodynamic and IBM models, and by adopting

the tuna farm off the southern Brač coast as the spawning location, we have valuable material

to assess both model enhancements. Higher horizontal resolution of the ROMS model would

allow for more detailed reproduction of circulation in a complex topographic area such as the

eastern Adriatic, with many structures like vortices, jets and wakes that may be important for

the advection of ichthyoplankton. More realistic introduction of swimming, diel vertical

migrations, and temperature related processes such as growth and mortality, and impact of

food availability in the IBM simulations would require new parameterizations based on the

laboratory and field experiments for the Adriatic.

Supporting information

S1 Table. Age and length of measured fish.

(XLSX)

S2 Table. Time series of seawater temperature measured at 1 m depth in the fish farm cage

near Ugljan.

(TXT)

S1 Appendix. Description of the hydrodynamic model and verification of its results.

(DOCX)

S1 Fig. Age frequency distribution of Atlantic bluefin tuna, Thunnus thynnus (Linnaeus,

1758), bullet tuna, Auxis rochei (Risso, 1810) and little tunny, Euthynnus alletteratus,

Spillover of the Atlantic bluefin tuna from cages in the Adriatic Sea

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Rafinesque, 1810 collected with commercial purse seiners operating in the southeastern

Adriatic, September 2011.

(TIF)

Acknowledgments

We are thankful to the INGV physical oceanography group that maintains Adriatic Forecast-

ing System (AFS) for providing boundary fields from their AREG model. Drifter data were col-

lected and made freely available by the International Argo Program and the national programs

that contribute to it (http://www.argo.ucsd.edu, http://argo.jcommops.org). The Argo Pro-

gram is part of the Global Ocean Observing System. Satellite SST data and Mediterranean

model results were obtained from E.U. Copernicus Marine Service Information.

Author Contributions

Conceptualization: Tomislav Dzoić, Gordana Beg Paklar, Barbara Zorica, Tanja Segvić-Bubić,

Vanja Čikes Keč, Ivona Mladineo, Leon Grubisić.

Data curation: Ivana Lepen Pleić, Ivona Mladineo.

Formal analysis: Tomislav Dzoić, Branka Grbec, Barbara Zorica, Vanja Čikes Keč, Ivana

Lepen Pleić, Ivona Mladineo, Philippe Verley.

Funding acquisition: Tanja Segvić-Bubić, Ivona Mladineo, Leon Grubisić.

Investigation: Barbara Zorica, Vanja Čikes Keč, Ivana Lepen Pleić, Ivona Mladineo, Leon

Grubisić.

Methodology: Tomislav Dzoić, Philippe Verley.

Project administration: Barbara Zorica, Vanja Čikes Keč.

Resources: Barbara Zorica, Tanja Segvić-Bubić, Vanja Čikes Keč, Ivona Mladineo, Leon

Grubisić.

Software: Tomislav Dzoić, Gordana Beg Paklar, Stjepan Ivatek-Sahdan, Philippe Verley.

Supervision: Gordana Beg Paklar.

Validation: Tomislav Dzoić, Gordana Beg Paklar, Barbara Zorica, Tanja Segvić-Bubić, Vanja

Čikes Keč, Ivana Lepen Pleić, Ivona Mladineo, Leon Grubisić.

Visualization: Tomislav Dzoić, Branka Grbec, Barbara Zorica, Tanja Segvić-Bubić, Vanja

Čikes Keč, Leon Grubisić.

Writing – original draft: Tomislav Dzoić, Gordana Beg Paklar, Branka Grbec, Barbara Zorica,

Tanja Segvić-Bubić, Vanja Čikes Keč, Ivona Mladineo, Leon Grubisić.

Writing – review & editing: Tomislav Dzoić, Gordana Beg Paklar, Barbara Zorica, Tanja

Segvić-Bubić, Vanja Čikes Keč, Ivona Mladineo.

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